Primary plant cell walls contain approximately 15-20% hemicellulose in the vegetative tissue and 40-60% in the grain.
Hemicellulose in monocot walls consists mainly of arabinoxylan (also referred to as glucurono-arabinoxylan or pentosans), a branched polymer consisting of a beta-gylosyl backbone decorated with arabinosyl and glucuronosyl residues. This polymer is often referred to as arabinoxylan because of the relatively low proportion of glucuronosyl residues. Carpita, N. C., “Structure and Biogenesis of the Cell Walls of Grasses” Annual Review of Plant Physiol. and Plant Molecular Biology 476:445-476 (1996).
Dicot cell walls contain xyloglucan as a major hemicellulosic polymer. Xyloglucan is also a branched polymer consisting of a linear beta-glycosyl backbone decorated with xylosyl residues, some of which are substituted with galactosyl residues.
Cell walls of corn grain constitute about 6-8% of the total dry weight of the seed. Whistler, R. L. et al, Hemicelluloses In Industrial Gums: Polysaccharides and Their Derivatives, San Diego: Academic Press, pp. 295-308 (1993). The grain cell wall contains 45-65% arabinoxylan, the remainder being mainly cellulose. Arabinoxylans are considered to be anti-nutritional components of animal feed because they can absorb large amounts of water. This leads to increased viscosity and possible sequestering of other digestible feed components, such as starch and polypeptides, away from digestive enzymes.
Arabinoxylans are known to lower the food conversion ratio (FCR) of animal feed. Studies where fungal xylanase were included in cereal-derived feed showed improved FCR and weight gain in broilers and pigs. Veldman and Vahl, H. A., “Xylanase in broiler diets with differences in characteristics and content of wheat”, British Poultry Science 35:537-550 (1994). The observed improvement in FCR and weight gain was higher than expected from arabinoxylan breakdown and digestion alone. Thus, arabinoxylan appears to reduce the digestion of other feed components. An unresolved need exists for methods and compositions to reduce the concentration of arabinoxylan (and thus hemicellulose) in grain.
Cellulose microfibrils have the highest tensile strength of any of the other cell wall polymers. Increasing the cellulose/arabinoxylan ratio in the cell wall should lead to a harder pericarp and improved ability to handle the grain.
About 12% of the corn processed through wet-milling is fiber (see: Johnson L A, and Can J B (2003) Wet milling: the basis of corn biorefineries; in Corn: Chemistry and Technology. P J White and L A Johnson (eds). AACC, St. Paul, Minn. pp. 449-494). Of this 12%, approximately 4% is coarse fiber (mainly pericarp-derived) and the rest is fine fiber. While coarse fiber consists of approximately 9% starch, fine fiber can have up to 30% starch, constituting 2.5% of the total dry mass. The other major component of fine fiber is arabinoxylan, which makes up to 21 % of this fraction. Some starch can become intricately associated with arabinoxylan, making it difficult to extract during wet-milling. A need exists for a method to reduce the concentration of arabinoxylan (and thus hemicellulose) in grain to improve starch extractability.